The pilot was making a no-wind takeoff downstream from a 4,000-foot slow flowing river. The takeoff water run for the conditions was calculated at 1,300 feet. The pilot and passenger both reported a partial loss of engine RPM just prior to lift-off from the water. According to the pilot, there was insufficient stopping distance before reaching a river bank. The pilot rotated the airplane off the water below flying speed, cleared the bank, and settled back into the tundra. The pilot stated that a similar loss of engine RPM had happened before, but was considered transient.